1. Field of the Invention
This invention relates to light sources such as lasers which are used with optical fiber and more particularly to a circuit which cancels the echo that arises when the laser is connected to the fiber by a connector.
2. Description of the Prior Art
In my U.S. patent application Ser. No. 07/264,356 entitled "Power Control System For Laser" which was filed on Oct. 31, 1988 and is assigned to the same assignee as is the present invention (hereinafter "the U.S. patent application Ser. No. 07/264,356"), U.S. Pat. No. 4,958,926, there is described a power control system for a laser which may be used in an optical time domain reflectometer (OTDR). As described in the U.S. patent application Ser. No. 07/264,356 a laser module is directly connected to the optical fiber to be monitored by a coupler. The OTDR sends a pulse of light into the fiber and uses the backscattered light to determine the location of any faults on the fiber. The power control system described in the U.S. patent application Ser. No. 07/264,356 allows the OTDR to automatically compensate for changes in laser characteristics, and to automatically and accurately provide the location of the fault. The system accomplishes the latter result by controlling the bias and pulse currents into the laser and also by ensuring that the electrical signal representative of the backscattered light is controlled to have a certain amplitude range.
The OTDR described in the U.S. patent application Ser. No. 07/264,356 may be connected to the fiber through a connector, i.e. the coupler rather that being directly connected to the fiber is connected to the fiber through a connector. While the use of a connector allows for easy removal of the OTDR from the fiber, it may present a problem as the light reflected from the connector (also sometimes referred to as an echo) can have an intensity which is from 20 to 200 times the intensity of the backscattered light. The electrical signal representative of that light would then saturate the electronics associated with the detector of the control system and the electronics would not "see" the backscattered light.
One solution to the saturation problem described above is to use analog switches after the detector to blank the high amplitude signal arising from the connector reflected light. The use of such switches adds noise, which has a detrimental effect on the operation of the OTDR. The noise is due to the bias currents, stray capacitances etc. associated with the switches. Alternatively, either acoustic or electro-optic masking can be used just before the detector. Such masking involves the use of undesirable very high voltages and radio frequency modulation and adds several thousand dollars to the cost of manufacturing the OTDR. Therefore, predetector masking while solving the saturation problem introduces other undesirable factors which must be taken into account in the design of the OTDR.
Of course, the gain of the electronics associated with the detector can always be made smaller so that the reflected light from the connector does not saturate the electronics. While the reduction in gain solves the saturation problem it also causes the electrical signal representative of the backscattered light from the fiber to be so low in amplitude that it is very hard to distinguish from noise. In other words, a reduction in gain sufficient to solve the saturation problem makes the OTDR unusable for its intended purpose.
It is desirable that the solution to the saturation arising from the light reflected from the connector does not detrimentally affect the operation of the OTDR. It is also desirable that the solution not involve the use of high voltages and radio frequency modulation. It is further desirable that the solution not add substantially to the cost of manufacturing the OTDR. The circuit of the present invention meets the above requirements.